Method of encapsulating a magnet with polytetrafluoroethylene



Feb. 28, 1961 sHQNKA ET AL 2,972,784

METHOD OF ENCAPSULATING A MAGNET WITH POLYTETRAFLUOROETHYLENE 2Sheets-Sheet 1 Filed March 4, 1958 INVENTORS fianalsjifiwnhz, E dzuarciG5 0 Zea/6L Feb. 28, 1961 METHOD OF ENCAPSULATING A MAGNET WITHPOLYTETRAFLUOROETHYLENE Filed March 4, 1958 2 Sheets-Sheet 2 ZZ aINVENTORS.

fianczls i 57mm Z'du/czrd G Solecki METHOD OF ENCAPSULATING A MAGNETWITH POLYTETRAFLUOROETHYLENE Francis R. Shonira, Riverside, and EdwardG. Solecki,

Chicago, lllh, assignors to Arthur S. La Pine & Company, a corporationof Illinois Filed Mar. 4, 1958, Ser. No. 719,148

Claims. (Cl. 18-59) This invention relates to a method of encapsulatingmagnets.

One method of stirring liquid substances in containers, such as glassbeakers, is to dispose a magnet in the beaker on the bottom thereof andto provide a moving magnetic field below the beaker which causes themagnet to move within the beaker and stir or agitate the liquid. Such astirring method is finding increased popularity in laboratories. Aserious problem arises, however, in that in order to preclude reactionsbetween certain liquids and the magnetic material comprising the magnet,it is necessary to coat or encapsulate the magnet with an inertsubstance. This protective coating must be free of flaws as even theslightest passage therethrough may be sufiicient to permit contaminationof the liquid which may result in serious economic and time loss.

A material which has been found to be particularly well-suited for suchprotective coating, due to its inert characteristics, is Teflon(tetrafluoroethylene polymer). While Teflon has the desirablecharacteristics of inertness, it has a serious disadvantage in that itis diflicult to mold uniformly around the magnet without flaws. To avoidthis problem, one method of encapsulating the magnet has been to providea pair of preformed cup-shaped Teflon elements which are placed over theopposite ends of the magnets and fused together as by sintering. Thismethod has the disadvantages of distorting the Teflon elements so that agrinding finishing operation is necessary, and the fused joints arerelatively weak. Another method of encapsulating the magnet is toprovide a cupshaped Teflon preform having a depth approximately equal tothe length of the magnet and providing a plug to close the open end ofthe preform after the magnet is inserted thereinto. This arrangement hasthe same defeet as in the previously described arrangement, as there isan inherent weakness in the connection between the plug and thecup-shaped member often resulting in the popping out of the plugtherefrom.

To avoid this separation of the jacket elements, a method of molding theTeflon completely around the magnet has been devised wherein granulatedTeflon is disposed around the magnet and heated to an elevatedtemperature whereupon the Teflon fuses into a solid jacket. A seriousdisadvantage of this method is that during the preforming of thegranular Teflon around the magnet into the shape desired, some flow ofthe Teflon particles occurs in different portions thereof. This flowcauses weak sections or flaws to result in the jacket which renders theproduct unsuitable for use.

The principal feature of this invention is the provision of a new andimproved method of encapsulating an elongated element, such as a rodmagnet, in a body of Teflon.

Another feature is the provision of such a method wherein granularTeflon is compacted about the magnet during the preforming operation,the arrangement of the granular Teflon and the molding means being suchthat during the compressive preforming operation, the density Patent 0ice of the granular Teflon increases without substantial flow thereof.

Another feature of the invention is the provision of new and improvedmethod for effecting the novel arrangement of the granular Teflonrelative to the magnet to permit such non-flowing compression thereof.

Other features and advantages of this invention will be apparent fromthe following description taken in connection with the accompanyingdrawings wherein:

Fig. 1 is an isometric view of a lower core pin and a mold cavityassociated therewith for use in a preforming of the encapsulated magnet,the mold cavity being shown in a retracted position;

Fig. 2 is an isometric view of the lower core pin and mold cavity ofFig. 1 with the mold cavity in a performing position;

Fig. 3 is an isometric endview of the lower core pin and mold cavity asarranged in Fig. 2 and in operable association with a charge measuringand depositing means;

Fig. 4 is an isometric view of a magnet depositing means inserted intothe mold cavity to deposit the magnet on the top of a first charge ofTeflon therein, the retracted position of the magnet depositing meansbeing shown in dotted lines;

Fig. 5 is an isometric view similar to that of Fig. 3 but with thecharge measuring and depositing means arranged subsequent to thedepositing of a second charge of Teflon over the magnet in the moldcavity;

Fig. 6 is an isometric view of the assembled upper and lower core pinsand mold cavity with the Teflon charges and magnet therein, a pressureapplying member being shown fragmentarily;

Fig. 7 is a vertical section showing the arrangement of the upper andlower core pins upon completion of the compacting step;

Fig. 8 is an isometric view of the lower core pin and mold cavity in theretracted position, with the completed preform accessibly exposed;

Fig. 9 is a fragmentary isometric view of an oven means with thepreformed encapsulated magnets being arranged in trays for sintering inthe oven;

Fig. 10 is a longitudinal section of an encapsulated magnet; and

'Fig. 11 is a transverse section thereof.

The novel method of encapsulating an elongated element, such as a rodmagnet, as comprehended by our invention, may be seen by a consecutivereference to Figs; 1 through 9 of the drawings. Broadly, the methodcomprehends the provision of means defining a suitable elongated moldcavity, closing the lower end of the mold cavity, depositing a firstquantity of granular Teflon in the moldvcavity on the core pin in such away as to provide an increased depth of the Teflon at the opposite endsof the cavity, placing the rod magnet on the deposited Teflon in acentered position, depositing a second, similarly arranged quantity ofgranular Teflon over the magnet and first quantity in the mold cavity,inserting an upper core pin to abut the upper surface of the Teflon andurging the core pins relatively together to compact the granular Teflonabout the magnet in the cavity. The completed preform is then picked upand placed in a suitable oven where it is sintered to complete theencapsulation process.

More specifically, a lower core pin 10 is provided having a base portion11 and an upstanding pin portion 12 terminating in an upper. concave end13. A block 14 defines a mold cavity 15 which is transversely elongatedand has a horizontal cross section closely comparable to the externalhorizontal cross section of the core pin 12. Block 14 is verticallymovable on portion 12 of the lower core pin to permit end 13 to beexposed above block 14 (in a retracted position of the block as seen inFig. l), or to be disposed adjacent to the bottom of the mold cavity (inthe molding position of the block as seen in,Fig. 2). The horizontalcross section of core pin portion 12 (and, thus, of cavity 15 includes arectangular side portion defined by elongated parallel sides 16, andopposite semicircular ends 17.

As seen in Fig. 3, block 14 is retained in the raised, molding positionby suitable means, such as a spacer device 18, and the assembly isassociated with a charge measuring and depositing means generallydesignated 19. Means 19 comprises a stand 211 provided with ahorizontally extending, fixed plate 21 having its upper surface 21aflush with the upper surface 14a of the mold cavity block. A slide plate22 overlies fixed plate 21 and is movable under a scraper bar 23 to aposition overlying the mold cavity 15 in block 14. Slide plate 22 isprovided with a plurality of dumbell-shaped apertures 24, the walls ofwhich extend perpendicular to the flat plane of plate 22. Extending fromslide plate 22 is a handle 25 'for use in moving the slide plate from acharge receiving position overlying fixed plate 21 (as shown in fulllines in Fig. 3) to a depositing position wherein an aperture 24overlies mold cavity 15 (as shown in dotted lines in Fig. 3). To providethe proper charge, the granular Teflon is placed in apertures 24, whileslide plate 22 is overlying fixed plate 21, so as to completely fill theapertures. Any excess granular Teflon extending above the upper surfaceof the slide plate is scraped off by scraper bar 23 as the plate ismoved outwardly to the depositing position. Thus, an accurate amount ofgranular Teflon is measured within the apertures. As seen in Fig. 3,when the outer aperture 24a moves into anoverlying position relative tocavity 15, a measured Teflon charge 27 falls therefrom into the cavity.

Because of the dumbell shape of the apertures, wherein each aperture isprovided with an elongated, narrow mid-portion 24b and laterallyenlarged end portions 240, a substantially greater depth of charge 27results at ends 17 of cavity 15 as compared to the depth thereof between sides 16 of the cavity. When charge 27 has been properlydeposited, a cylindrical rod magnet 28 is placed within the cavity in acentered position on top of charge 27. The length of the cavity betweensides 16 is prefcrably comparable to the length of magnet 28 so that themagnet is substantially fully received between sides 16, ends 17 of thecavity extending outwardly beyond the opposite ends 23:: of the magnet.

As seen in Fig. 4, a magnet inserter 29 is utilized in placing themagnet on charge 27. Inserter 29 comprises a block 35) of non-magneticmaterial, suchas brass, having a transverse cross section similar to thecross section of cavity 15. At its inner end, the block is provided witha slot 30:: in which is received a keeper 31 formed of a magneticmaterial such as iron. The width of the groove is slightly less than,the width (diameter) of the rod magnet 28. The keeper is movablealternatively upwardly and downwardly in slot 30a by means of a rod 32extending outwardly from the block and provided at its outer end with aknob 33 and adjustable stop nuts 34 for limiting the inward movement ofthe rod. At the innermost limit of travel of rod 32, keeper 32 isdisposed just slightly within slot 30 so that the magnet 28 may contactthe keeper and be retained thereagainst by the induced magneticattraction.

To deposit magnet 23 oncharge 27, the inserter .29 is inserted intocavity 15 with magnet 28 retained .by keeper 31 in a center position atthe lower. endof the block. As the block has a close fit with the wallsof the cavity, the magnet is automatically properly centered. When themagnet is resting on top of. the charge 27, knob 33 is moved upwardlywhile the block is retained in place in the cavity. This separateskeeper 31 from the magnet a'sthemagnet can not follow the keeperupwardly into the groove due to the somewhat narrower width of thegroove. With keeper 31 maintained in the upwardly spaced'position, theentire inserter is removed from the cavity (as seen fragmentarily indotted lines in Fig. 4), leaving the magnet in a centered position ontop of charge 27.

A second charge 35 of Teflon is now deposited on top of first charge 27and magnet 28 resting on first charge 27. This is accomplished by movingslide plate 22 outwardly from stand 20 until a second aperture 240. isdisposed above cavity 15 permitting the measured Teflon charge thereinto drop into the cavity. As with charge 27 delivered from aperture 24a,an extra quantity of Teflon is deposited at the ends of the cavity tocompensate for the fact that the magnet does not extend there into.

The Teflon is now compacted around the magnet by inserting an upper corepin 36 into cavity 15 to abut the upper surface of second charge 35.Suitable means 37 are provided to apply a substantial pressure againstcore pin 36 to urge the core pin downwardly into the cavity and compactthe Teflon about the magnet. The particular means employed for providingthe pressure forms no part of this invention and, thus, requires nofurther description here other than to indicate that means 37 should becapable of building up the pressure slowly over a period of one minutefrom 0 pound per square inch to 7,000 pounds per square inch and holdingthis maximum 7,000 pounds per square inch pressure for approximatelyone-half minute to effect the necessary compacting preforming of theTeflon.

At its innermost travel in the compacting operation, upper core pin 36remains spaced slightly from lower core pin portion 12. The lower end 38of upper core pin 36 is concave complementarily to upper end 13 of thelower core pin, the concave ends 13 and 38 having generallysemi-cylindrical mid-portions 13a and 38a, respectively, and generallysegmentally spherical end portions 13b and 38a, respectively. Thus, theTeflon is formed by abutment with the ends 13 and 33 into a generallyrounded preform 41; However, between core pin ends 13 and 38, thepreform assumes the surface configuration of the cavity wall and, thus,flat, parallel surfaces 39 are formed which are diametrically opposedrelative to the axis of the magnet. Further, because of the fact thatthe core pins-are slightly smaller than the cavity to permit readymovement therebetween, surfaces 39 are disposed somewhat outwardly fromthe cylindrical configuration defined by the core pin ends, thusslightly projecting bosses 39a are formed which have as their outermostends the flat surfaces 39. Because of the circular nature of ends 17 ofcavity 15, the side surfaces 39 are joined at the ends of the preform bycylindrical surfaces 40.

Upon completion of the pressure application step, upper core pin 36 isremoved and spacer means 18 is withdrawn to permit the mold cavity block14 to move back to the retracted position where it rests on base 11 ofthe lower core pin. This'disposes the completed preform 41 upwardly ofupper surface 14a of the mold cavity block allowing the preform 41 to bepicked up from its position on end 13 of the lower core, and placed on asuitable tray, such as tray 42, for insertion into an oven 43 whereinthe preform is sintered to complete the encapsulation process. It shouldbe noted that this sintering occurs without confinement of the preform(no molding means are, retained around the preform during the sinteringoperation) while yet the compacted structure of the preform maintainsthe shape thereof very accurately. In the sintering of the preform, thetemperature is raised over a period of one and oneehalf hours toapproximately 720 F., which temperature is maintained for approximatelyone-half hour. The temperature is then lowered to approximately 500" F.within one hour, and then to room temperature as desired. The resultantencapsulated magnet is very closely'similar in size to the preform 41.As no flaw inducing stresses .are'developed in the Teflon during thepreforming process, such as might be caused by a flow of the Teflonduring the preforming process, the resulting encapsulated magnet 44 hasbeen found to be substantially free of flaws or defects. As theencapsulation is uniformcompletely around the magnet, there is notendency for different portions thereof to separate such as whereseparable jacket elements are fused together to effect theencapsulation.

By providing the flat surfaces 39, the encapsulated magnets 44 areprevented from having a free rolling action on flat surfaces, such ascounter tops, substantially facilitating their use. Further, because ofthe projecting boss structure 39a, an additional roll preventingstructure is provided as well as a structure which imparts an improvedstirring action due to the fact that the encapsulated magnet does notpresent a completely symmetrical rounded exterior.

While we have shown and described certain embodiments of our invention,it is to be understood that it is capable of many modifications.Changes, therefore, in the construction and arrangement may be madewithout departing from the spirit and scope of the invention as definedin the appended claims.

We claim:

1. The method of preforming a Teflon encapsulated rod magnet comprisingin combination, the steps of: depositing a first quantity of granularTeflon in a mold cavity so as to provide an increased height of theupper surface of the Teflon at opposite portions of the cavity;disposing a rod magnet on the deposited Teflon in a centered position toextend between said opposite portions in the cavity; depositing a secondquantity of granular Teflon over said first quantity and magnet in themold ca ity, the depositing of the second quantity being controlled todeliver a greater amount of the Teflon to opposite portions of thecavity than to the portion therebetween; and urging means relativelytogether in the cavity to compact the Teflon about said magnet withoutsubstantial flow thereof.

2. The method of preforming a Teflon encapsulated rod magnet comprisingin combination, the steps of: providing means defining a transverselyelongated mold cavity having a closed lower end; depositing a firstquantity of granular Teflon on said end so as to provide an increasedheight of the upper surface of the Teflon at the transverse ends of thecavity; disposing a rod magnet on the deposited Teflon in a centeredposition transversely of the cavity to extend between said increasedheight portions of the Teflon upper surface; depositing a secondquantity of granular Teflon over said first quantity and magnet in themold cavity, the depositing of the second quantity being controlled todeliver a greater amount of the Teflon to opposite portions of thecavity than to the portion therebetween; inserting a core pin throughthe upper end of the mold cavity to abut the upper surface of saidsecond quantity of Teflon; and urging said core pin relatively towardsaid lower end to eflect'a predetermined minimum spacing therebetweenthereby to compact the Teflon about said magnet without substantial flowthereof.

3. The method of preforming a Teflon encapsulated rod magnet comprisingin combination, the steps of: providing means defining a transverselyelongated mold cavity having a closed lower end; depositing a firstquantity of granular Teflon on said end so as to provide an increasedheight of the upper surface of the Teflon at the transverse ends of thecavity; placing a rod magnet against a magnetic carrier; associating astripper with the carrier; inserting the carrier with the magnetlowermost thereon through the upper end of the mold cavity to disposethe magnet on the deposited Teflon in a centered position transverselyof the cavity to extend between said increased height portions of theTeflon upper surface; effecting relative movement of the carrier andstripper to separate the carrier from the magnet; withdrawing thecarrier and stripper from the mold cavity; depositing a second quantityof granular Teflon over said first quantity and magnet in the moldcavity, the depositing of the second quantity being controlled todeliver a greater amount of the Teflon to opposite portions of thecavity than to the portion therebetween; inserting a core pin throughthe upper end of the mold cavity to abut the upper surface of saidsecond quantity of Teflon; and urging said core pins relatively towardsaid lower end to compact the Teflon about said magnet Withoutsubstantial flow thereof.

4. The method of preforming a Teflon encapsulated rod magnet comprisingin combination, the steps of: providing means defining a transverselyelongated mold cavity; closing the lower end of the mold cavity with alower core pin; depositing a first quantity of granular Teflon in themold cavity on the core pin so as to provide an increased height of theupper surface of the Teflon at the transverse ends of the cavity toextend between said increased height portions of the Teflon uppersurface; disposing a rod magnet on the deposited Teflon in a centeredposition transversely of the cavity; depositing a second quantity ofgranular Teflon over said first quantity and magnet in the mold cavity,the depositing of the second quantity being controlled to deliver agreater amount of the Teflon to opposite portions of the cavity than tothe portion therebetween; inserting an upper core pin through the upperend of the mold cavity to abut the upper surface of said second quantityof Teflon; urging said core pins relatively together to compact theTeflon about said magnet into a preform element without substantial flowof the Teflon; removing the preform element from the mold cavity; andsintering the unenclosed preform.

5. The method of preforming a plastic encapsulated elongated elementcomprising in combination, the steps of: providing means defining a moldcavity; closing the lower end of the mold cavity; depositing a firstquantity of moldable plastic material in the mold cavity so as toprovide an increased height of the upper surface of the plastic atopposite portions of the cavity; disposing an elongated element on thedeposited plastic in a centered position in the cavity to extend betweensaid opposite portions of the cavity; depositing a second quantity ofthe moldable plastic material over said first quantity and element inthe mold cavity, the depositing of the second quantity being controlledto deliver a greater amount of the Teflon to opposite portions of thecavity than to the portion therebetween; inserting a force applyingmeans through the upper end of the mold cavity to abut the upper surfaceof said second quantity of plastic material; and urging said forceapplying means toward the lower end of the cavity to compact the plasticmaterial about said element without substantial flow of the plasticmaterial.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Mold Loading Methods, Plastics World, March 1947, page 3.

Teflon, Du Pont Information Bulletin No. X-7c, en-

bulletin, 12 pp., pages 4 and 6 relied upon, July 7. Noftsinger:Abstract of pat. appl. 697,592, 0.6., vol. 645, page 328, Apr. 3, 1951.

1. THE METHOD OF PREFORMING A TEFLON ENCAPSULATED ROD MAGNET COMPRISINGIN COMBINATION, THE STEPS OF: DEPOSITING A FIRST QUANTITY OF GRANULARTEFLON IN A MOLD CAVITY SO AS TO PROVIDE AN INCREASED HEIGHT OF THEUPPER SURFACE OF THE TEFLON AT OPPOSITE PORTIONS OF THE CAVITY,DISPOSING A ROD MAGNET ON THE DEPOSITED TEFLON IN A CENTERED POSITION TOEXTEND BETWEEN SAID OPPOSITE PORTIONS IN THE CAVITY, DEPOSITING A SECONDQUANTITY OF GRANULAR TEFLON OVER SAID FIRST QUANTITY AND MAGNET IN THEMOLD CAVITY, THE DEPOSITING OF THE SECOND QUANTITY BEING CONTROLLED TODELIVER A GREATER AMOUNT OF THE TEFLON TO OPPOSITE PORTIONS OF THECAVITY THAN TO THE PORTION THEREBETWEEN, AND URGING MEANS RELATIVELYTOGETHER IN THE CAVITY TO COMPACT THE TEFLON ABOUT SAID MAGNET WITHOUTSUBSTANTIAL FLOW THEREOF.